Field of the Invention
The present invention relates to an organic electroluminescent device, a display apparatus, and a lighting apparatus.
Description of Related Art
An organic electroluminescent device (also referred to as an organic EL device, hereinafter) is a device having a luminescent layer which contains a light-emitting compound held between a cathode and an anode. The organic EL device emits light by injecting electrons and holes into the luminescent layer, allowing them to recombine therein so as to produce excitons which cause emission of light (fluorescence or phosphorescence) upon annihilation. The device attracts public attention to a flat display apparatus or a lighting apparatus of the next generation, by virtue of its capability of luminescence under applied voltage of only several volts to several tens of volts.
Since an organic EL device making use of phosphorescence from excited triplet state, where the upper limit of internal quantum efficiency reaches 100%, has been reported by a team of Princeton University (see M. A. Baldo et al., Nature, Vol. 395, p. 151-154 (1998), for example), materials emitting phosphorescence at room temperature has been more extensively studied (see M. A. Baldo et al., Nature, Vol. 403, No. 17, p. 750-753 (2000), and U.S. Pat. No. 6,097,147, for example.).
On the other hand, heavy metal complexes, such as iridium complexes or the like, have been investigated as materials emitting phosphorescence at room temperature.
For example, tris(2-phenylpyridine) iridium complex has been widely known (see M. A. Baldo et al., Nature, Vol. 403, No. 17, p. 750-753 (2000)). Moreover, an iridium complex having ligands composed of a tris(2-phenylpyridine) skeleton introduced with silyl groups, aimed at improving durability of dopant and luminescence efficiency, has been disclosed (see Japanese Laid-Open Patent Publication No. 2005-327526, for example).
However, the organic EL devices making use of dopants have been still unsuccessful to achieve satisfactory levels of performances, represented by limited luminescent lifetime.
Iridium complexes other than tris(2-phenylpyridine) iridium complex disclosed include those having phenylimidazole ligands and carbene ligands (International Patent Nos. WO2006/046980 and WO2005/019373, for example).
These materials have been suffering from problems in that they have shallower HOMO as compared with complexes of phenylpyridine ligands and larger barrier height against hole injection. Moreover, the molecules thereof would need larger reorientation energy if introduced with bulky substituents aimed at attaining luminescence at shorter wavelengths, the charge transfer performance would degrade, and the drive voltage would elevate as a consequence.
On the other hand, complexes having ligands of iridium complex linked thereto have been disclosed (see International Patent Nos. WO2005/76380 and WO2004/81017).
While these complexes are intrinsically improved in their thermal stability, and lifetime of the applied devices is consequently improved, the publications disclose nothing about improvement in the drive voltage particularly in an effort of obtaining luminescence at shorter wavelengths, using materials having shallower HOMO.